In the chemical process and petroleum refining industries, maintenance and often capital improvements of process plants occur on a scheduled basis, either annually, biannually, or less often, if possible, at scheduled times when the operating unit is completely shut down and every aspect of its operation inspected and maintained. Operating units are made up of various types of vessels from tanks to heat exchangers to distillation columns, catalytic reactors and the like. This massive maintenance effort on each unit is called a turnaround and maximum speed in getting the unit back on stream is the order of the day, since the main cost of such an operation, even though extensive, is the time during which this operating unit is not producing product or refining petroleum, which can be sold. The speed at which the jobs are done are, of course, tempered with the necessity that safety of the workers be maintained throughout the operation.
During a turnaround, an operating process vessel cannot merely be shut down and drained before entry of maintenance and repair crews. The vessel must first be rendered safe and decontaminated of any residual material in it which may be harmful, even fatal, to the workers who must enter to inspect and maintain, if not refurbish, the internal structure of the vessel. The present practice, depending upon the vessel, is to inject steam for a period of time until monitoring devices indicate that no dangerous gases remain which present an explosive hazard to workers who must work in this environment. Vessels are also washed with water to remove contaminants where applicable, and often, both steaming and water washing is involved in the degassing of a vessel to make it possible for humans to safely enter to inspect and repair. Hazardous contamination of process vessels may also come from auxiliary pieces of equipment which are attached in fluid communication such as, for example, various kinds of heat exchangers and demister pads. This problem has been made much more difficult in view of the present concern that the hazardous materials removed from the vessel being cleaned become an environmental problem, either through release to the atmosphere or through improper disposal of wash water containing the waste. The previous attempts at steaming were just that, attempts, and often required long periods of time, days even, before the "sniffers" indicated the contaminant had been either removed or reduced to such a minuscule presence that the vessel was safe for entry. Water washing results in an inordinate amount of liquid waste for which disposal must be accomplished.
Of particular importance is the removal of benzene and other volatile organic carbons from the process vessels. It has been long recognized that chronic exposure by humans to benzene at high levels in the chemical and petrochemical work place leads to bone marrow depression, aplastic anemia, and leukemia. Although absorption of benzene across the skin as a vapor or in aqueous media is minimal, benzene toxicity is most frequently caused by inhalation of benzene through the respiratory system. Present government safety standards for eight hour work days are set at 1.0 ppm (average) benzene. The National Institute For Occupational Safety And Health (NIOSH) has recommended (1989) an occupational long term exposure limit in air of 0.1 ppm benzene. That is not good enough since workmen involved in cleaning process vessels are exposed on a year-round basis to the interior of these vessels. The goal of the present invention, therefore, is to provide a process for vessel decontamination which exceeds this standard--in fact, which approaches, if not meets, 0 ppm of benzene. Benzene can be trapped beneath scale or other contaminants only to seep out at a later time when cleaning had been considered completed or gather in the head space of the vessel.
Even though there are many conflicting problems which converge on the process vessel, still, the overriding requirements are speed with safety, and the practice of this invention over the present practice accomplishes these while providing avenues for a more effective protection of the environment. This invention allows almost pristine cleaning of process vessels without even opening them in many cases until safely decontaminated of dangerous substances, such as benzene.